It is very important that a rifle scope be installed on a rifle so that when the rifle is level, the vertical line of the scope reticle is aligned plumb (i.e., perpendicular) relative to the Earth. This relationship ensures that when the scope's elevation dial is adjusted the resultant changes in bullet impact points will coincide with the direction of bullet drop, which is determined by gravity. If the scope is installed such that when the rifle is level the vertical line of the scope reticle does not mirror the direction of bullet drop, a windage (or horizontal) error is progressively introduced as the scope's elevation dial is adjusted for greater bullet drop. Despite this need, there appears to be no fixture adapted to facilitating scope installation so that this requirement is met.
During scope installation, a rifle is typically placed in a cradle or vise. Ideally, the rifle is oriented so that the top of the receiver (or the scope base attached thereto) is level when evaluated in a direction perpendicular to the long axis of the rifle (transversely level). Unfortunately, maintaining this orientation until scope installation is complete is problematic. Cradles normally don't provide a means of securing the rifle in the proper position, and vises can disturb the rifle as they are tightened. These shortcomings frequently result in the expenditure of time and energy, as multiple attempts to maintain and verify the rifle's proper orientation are required. Worse yet, a frustrated installer may, at some point, intentionally or not, mount the scope on an improperly positioned rifle.
When a scope has been properly installed and the rifle is held level, as the scope's elevation dial is adjusted bullet impact at a fixed range will vary, but it will do so strictly in conformance with the direction of bullet drop. However, when a scope has been installed so that, when the rifle is level, the vertical line of the scope reticle is not plumb, a windage (horizontal) error is introduced. That is: as the elevation dial is adjusted, bullet impact is moved laterally, as well as vertically. This built-in windage error would not be evident to the shooter, and it would lead to decreased shooting accuracy, especially at longer shooting distances.